1. Field of the Invention
The invention generally relates to a purification system, and in particular relates to a method for purifying corrosive liquefied gases of metal impurities.
2. Brief Description of the Related Art
The purification of vapor phase impurities has been described for electronic specialty gases such as HCl. These methods have taken advantage of the gaseous purification features predicted by the phase diagram equilibrium thermodynamics. Additional methods have used chemisorption of gaseous impurities (Tsvetn. Met, (2), 67-71, 1995), solvent extraction of organic compounds (Solvent Extr. Ion Exch. 11(1), 239-57, 1993), and use of activated carbon for removal of C1-C3 chlorinated hydrocarbons (Japan Kokai Tokkyo Koho, JP 03265503 A2), for example.
A trend in Very Large Scale Integration ("VLSI"), e.g., integrated circuit ("IC"), technology is the requirement for very clean reagents used in chemical processes to eliminate defects in the IC caused by impurities diffusing into the semiconductor bulk. This increasing demand with regard to purity levels is becoming more stringent, requiring in some cases very low part-per-billion (ppb) levels of metals. These metals include, but are not limited to, aluminum, calcium, cobalt, sodium, zinc, iron, nickel, chromium, molybdenum, copper, manganese, magnesium, and arsenic.
Semiconductor manufacturing processes have turned to using ultra-high purity corrosive liquefied gases such as BCl.sub.3, HBr, HCl, and Cl.sub.2, for dry etching and cleaning. These gases are typically delivered to the user in a steel or stainless steel, internally Ni-phosphide-coated cylinder or container which contains the liquid-phase corrosive gases. Semiconductor manufacturers require that the corrosive gases be substantially free of metallic impurities, and as a result, higher purity is required in both gas and liquid phases.
It is known that the liquid phase of corrosive gases usually contains much higher levels of metal impurities in the cylinder. See "Analysis of hydrogen chloride for metals contamination", Institute of Environmental Sciences 1996, proceeding by BOC; "What is the shelf life of electronics specialty gases?", Institute of Environmental Sciences 1996, proceeding by Air Products and Chemicals, Inc. This is because many metal compound impurities which are included in corrosive liquefied gases have lower vapor pressures than the corrosive matrix gases. The metals can be in particle form or dissolved in the liquid phase of the gas. Some of these metal impurities originate from the source product, and some come from corrosion of the filling system or from the cylinder/container during storage.
Normally, high purity liquified corrosive gases are purified by classical distillation to remove some impurities and stored in a metal-based storage tank from which it is transferred into the final cylinders/containers in their liquid phases at the production site. However, such liquid-phase filling of the final cylinder/container can introduce metallic impurities from the production facilities into the final cylinder.
It is therefore an object of the present invention to provide a more efficient method and apparatus for the removal of metal impurities and metal contamination of liquified gas during gas-phase filling of an intermediate or the final cylinder/container.
It is another object of the present invention to provide an economical method to effectively purify liquified corrosive gas of metal impurities, immediately followed by filling of the final cylinder/container with the purified gas.